Various alarm devices have been proposed heretofore which produce an audible or other alarm signal when a person touches the outside doorknob of a door leading into the premises to be protected.
Examples of such prior proposals in which the alarm apparatus is A.C. powered are disclosed in my U.S. Pat. No. 4,011,554, "Popular Electronics" magazines, Feb. 1969, pp. 92-93, U.S. Pat. No. 3,508,239 to John V. Fontaine, and U.S. Pat. No. 3,465,325 to Goldfarb et al.
Examples of such prior proposals in which the alarm apparatus is battery powered are disclosed in U.S. Pat. No. 3,623,063 to John V. Fontaine, and in "101 Electronic Projects For Under $15", a Davis Publication, 1975 edition, pp. 56-57. The previous battery powered intrusion alarms had two principal disadvantages: short battery life, and frequent false alarms caused by an inability to reliably discriminate between the capacitance of a person's body and various stray capacitances.